As Far as "smaller frames" ( midsize frames) go-- I hate them--- they tend to be a hybrid design of an lr308 mixed with ar15 -- BUT they are all proprietary and compatibility of parts is horrible -- i did one, and sold it as proprietary parts were always hard to get or just flat out not available
You can build lightweight large frame ar's pretty easy...my 308w weighs 5.8 pounds with 3-9 optic for $1000 Just gotta do some homework and search "ultralight" parts...and be willing to spend some cash to drop ounces. It's a numbers game so look at dropping grams and ounces-- it all adds up to pounds.
Look for titanium, magnesium, and carbon fiber parts. V7 sells small parts, tool craft has a ti bolt for a decent price, faxon has multiple profile barrels that shoot good. Smokes composites stocks, briggand arms handguards. It's another rabbit hole to fall down. Good luck.
Every part will add up so even the smallest thing matters when it comes to ultralight.
I probably could have dropped another 1/2 pound or more if i wanted to buck up with another $1-1500 for lighter parts
